As is well known in the related art, a liquid crystal display device includes two substrates and liquid crystal sealed therebetween. In the liquid crystal display device, a voltage is applied between electrodes provided on the two substrates to drive the liquid crystal in each pixel, thereby changing the optical property of the liquid crystal. In this way, the liquid crystal display device controls the transmission and shielding of light to display an image on a screen.
In general, a substrate having a glass plate as a base and liquid crystal driving electrodes provided on the base has been used as the above-mentioned substrate. The electrode provided on one of the two substrates is transparent. For example, an ITO thin film is used as the electrode. The electrode provided on the other substrate is changed depending on a liquid crystal driving type. For example, a plurality of transparent electrodes or reflecting electrodes is arranged for each pixel, and a plurality of TFTs is connected to each of the plurality of electrodes.
Color filter films that color display light may be provided on these substrates. The color filter film includes a plurality of coloring films that is arranged so as to correspond to the pixels. For example, red (R), green (G), and blue (B) coloring films are arranged so as to correspond to the pixels. It is possible to display a color image on the screen by using a substrate including the color filter film. The substrate including the color filter film is sometimes referred to as a “color filter”.
Therefore, these substrates of the liquid crystal display device are manufactured through a plurality of complicated processes, such as a process of cleaning the surface of a glass plate, a process of forming an electrode film, a process of patterning the electrode film, a TFT forming process, and a color filter film forming process.
Each of the manufacturing processes of the substrate is generally performed in a clean room. Nonetheless, a fine foreign material is likely to be adhered to the substrate during any one of the complicated manufacturing processes. It is considered that the foreign material is caused by dust in the atmosphere of the clean room.
The foreign material adhered to the substrate is likely to damage the surface of an opposite substrate when the liquid crystal display device is assembled. Therefore, the substrate having the foreign material adhered thereto is excluded as a defective product from a liquid crystal display device assembly process and is then corrected, or the glass base is collected and reproduced, or discarded.
However, it has been known that, among the foreign materials, a foreign material with a diameter of 30 μm or more damages the opposite substrate. In addition, a foreign material with a diameter of 20 μm or more is also likely to damage the opposite substrate depending on the kind of liquid crystal display devices manufactured. Therefore, when the size of the foreign material is smaller than the above-mentioned value, the foreign material is ignored during the assembly of the liquid crystal display device without being removed.
However, these foreign materials may include a conductive material. When a substrate having the conductive foreign material adhered thereto is used to assemble the liquid crystal display device, the electrodes of two substrates opposite to each other are short-circuited, making it difficult to normally display an image on the screen. This short circuit occurs even when the diameter of the conductive foreign material is smaller than 20 μm.
When the foreign material adhered to the substrate is a conductive material, the substrate with the foreign material is excluded from the liquid crystal display device assembly process regardless of the diameter thereof. When the foreign material adhered to the substrate is a non-conductive material, it is necessary to exclude the substrate with only the foreign materials with a diameter of more than 20 μm or 30 μm from the liquid crystal display device assembly process. If the substrate having a foreign material with a small diameter adhered thereto is excluded without discriminating conductive and non-conductive materials, non-defective substrates are also excluded, which results in a significant reduction in yield. If the substrate having a foreign material with a small diameter adhered thereto is used during the liquid crystal display device assembly process without discriminating conductive and non-conductive materials, the assembled liquid crystal display device becomes a defective product, which results in an increase in loss.
However, it is very difficult to test whether the fine foreign material is a conductive material. It is considered that the conductive foreign material is also caused by dust in the atmosphere of the clean room. Therefore, the surface of the conductive foreign material is likely to be oxidized. In the liquid crystal display device assembly process, it is assumed that the oxide film on the surface of the foreign material is damaged and a conductive inner part is exposed, which causes a short circuit. Therefore, it is difficult to test whether there is a foreign material during a substrate manufacturing process before the liquid crystal display device assembly process.
In addition, the test needs to be rapidly and effectively performed without delaying the substrate manufacturing process. If it takes a long time to perform the test, the test process becomes a rate-controlling process, and the overall substrate manufacturing process is delayed significantly.
For example, Patent Documents 1 and 2 disclose a method of testing whether a foreign material is adhered to a substrate or the height of the foreign material. However, it is impossible to test whether there is a conductive foreign material using this method.
In addition, Patent Document 3 discloses a method of testing a short circuit between the electrodes after a liquid crystal display device is assembled. However, in this method, the entire short-circuited liquid crystal display device is treated as a defective product.    [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-177192    [Patent Document 2] Japanese Patent Application Laid-Open No. 2006-300892    [Patent Document 3] Japanese Patent Application Laid-Open No. 11-73132